The Institute of Electrical and Electronics Engineers (IEEE) has developed a network communication standard 802.1d entitled, "Media Access Control (MAC) Bridges," approved May 31, 1990 (hereinafter "IEEE 802.1d"). In brief, IEEE 802.1d describes functions to be performed by compliant bridges including a Spanning Tree Protocol, building and maintenance of a filtering database, relaying and filtering of frames, and transmission of Bridge Protocol Data Units (BPDUs). A BPDU is a special message that bridges transmit to each other to facilitate determination of a spanning tree. The Spanning Tree Protocol's primary function is to prevent loops in the bridged network. The Spanning Tree Protocol does this by enabling and disabling certain ports on a network device based upon negotiations with neighboring devices.
Several limitations of learning bridges in the prior art have been observed by the assignee of the present invention. For example, a port's transition from the blocked state to the non-blocked state may cause attached subnets to be flooded with packets that are addressed to nodes belonging to those subnets. This flooding results in less than efficient operation of the learning bridge. Additionally, the conventional port-level approach to the Spanning Tree Protocol employed by prior art learning bridges is incompatible with the concept of trunking.
Generally, trunking can be thought of as a means of providing bandwidth aggregation between two points in a network (e.g., between two network devices). The multiple physical network links coupling two devices, for example, may be combined to form one logical channel, referred to as a "trunk," between the first device and the second device. As far as the Spanning Tree Protocol is concerned, a trunk is a loop. Thus, the conventional port-level approach will block at least N-1 ports of an N-port trunk to eliminate the perceived loop.
Based on the foregoing, it is desirable to provide a network device that provides more intelligent spanning tree processing. Specifically, it is desirable to perform the Spanning Tree Protocol at a trunk-level rather than at the port-level, such that all ports of a trunk are treated in a like manner with respect to the Spanning Tree Protocol. It is also desirable to provide one or more intermediate states to facilitate a port's transition from the blocked state to the non-blocked state to reduce flooding. Further, it would be advantageous to provide improved methods of BPDU reception and transmission.